<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      .box {
        width: 5rem;
        height: 100px;
        background: red;
      }
    </style>
    <script></script>
  </head>

  <body>
    <div class="box">111</div>
    <script>
      window.onresize = function () {
        var w = window.innerWidth;
        document.documentElement.style.fontSize = w / 10 + "px";
      };
    </script>
  </body>
</html>
